The 7.5” x 12” Hi-Q is a dual field coil with a fancy new name. A large outer coil with a small inner coil. If you use a small test nugget only the small inner coil is reacting. The larger outer coil adds more depth for larger targets. The 12” round dual field will act the same way. Only the inner coil will respond to small targets, the larger outer coil the larger targets. As a round coil it has a larger total area than the Hi-Q elliptical so will get just a little more depth on large